The present invention relates to a motor-driven compressor having a metal housing, an electric motor housed in the metal housing and a compression unit also housed in the metal housing and compresses refrigerant by being driven by the electric motor, wherein a neutral point of the electric motor is formed by a connecting portion that connects core wires of polyphase coils of the electric motor.
Metal housing of a motor-driven compressor generally houses therein not only an electric motor but also a compression unit compresses refrigerant by being driven by the electric motor. In such a motor-driven compressor, the refrigerant circulates through the compressor in gaseous form during its normal operation. When the compressor is stopped and the gaseous refrigerant in the housing is cooled, however, the refrigerant may be liquefied and collected in the housing in the form of a liquid. The liquid refrigerant has a lower specific resistivity than the gaseous refrigerant. Though varying depending on the kind of oil, some oils used to lubricate the motor-driven compressor may reduce its specific resistivity by being mixed with the liquid refrigerant.
If any conductive portion of the electric motor that is exposed to the interior of the housing is put in the liquid refrigerant collected in the housing, therefore, the conductive portion and the housing are electrically conducted through the liquid refrigerant. In addition, insulation resistance between the conductive portion and the housing is reduced by a decrease of the specific resistivity of the liquid refrigerant. If the motor-driven compressor with a decreased insulation resistance is started, there is a fear that electric current supplied to the conductive portion may leak to the housing through the liquid refrigerant and oil.
Japanese Unexamined Patent Application Publication No. 2001-182655 proposes a motor-driven compressor according to which the insulation resistance between the conductive portion and the housing through the liquid refrigerant is increased. The motor-driven compressor includes a feeder terminal as the above conductive portion that is held by the base of a metal housing through an insulating member. The feeder terminal and the insulating member that are located inside the metal housing are covered with an insulating resin tube.
The insulating resin tube covers the feeder terminal with one end of the insulating resin tube set in close contact with the insulating member. If liquid refrigerant is collected in the metal housing, leakage current between the feeder terminal and the base flows through the liquid refrigerant having the lowest specific resistivity therebetween. Thus, the insulation resistance between the feeder terminal and the base depends on the creepage distance along the insulating resin tube and the insulating member and on the area of the passage of the liquid refrigerant through which the leakage current flows. Because the insulating resin tube is set in close contact with the insulating member, the creepage distance is higher as compared to the case where no such insulating resin tube is provided, so that the insulation resistance is increased.
The electric motor of a motor-driven compressor includes three-phase coils having a U-phase coil, a V-phase coil and a W-phase coil. The ends of the three-phase coils are drawn out therefrom and connected together to form a connecting portion as the conductive portion, which provides a neutral point of the electric motor. Japanese Unexamined Patent Application Publication No. 2005-278289 discloses a neutral terminal device of a rotating electric machine which is designed to prevent a short circuit between the conductive wires of the coils of the rotating electric machine and the ends of the neutral lines of the coils. The neutral terminal is formed so that the ends of the neutral lines do not project out from a metal sleeve and the neutral terminal is covered with an insulation cap.
However, if the connecting portion of the coils as the neutral point is put in the liquid refrigerant, the connecting portion and the housing are electrically conducted through the liquid refrigerant. In addition, the insulation resistance between the connecting portion and the housing is reduced by a decrease of the specific resistivity of the liquid refrigerant. As a result, there is a fear that electric current supplied to the connecting portion may leak to the housing through the liquid refrigerant. However, Japanese Unexamined Patent Application Publication No. 2001-182655 discloses no insulation between the neutral point of the electric motor and the housing.
In order to reduce the production cost of an electric motor, the neutral point is generally formed in such a way that the length of the conductive wire that is drawn out from the coil and the length of the connecting portion are as small as possible. In addition, to prevent the neutral point from interfering with the housing or the coil, the neutral point is formed as short as possible. Merely covering the neutral terminal as the neutral point with the insulation cap as in the case of Publication No. 2005-278289 only provides poor insulation resistance between the neutral terminal and the housing when the liquid refrigerant enters into the insulation cap and the neutral terminal is put in the liquid refrigerant.
The present invention, which has been made in light of the above problems, is directed to a motor-driven compressor wherein insulation resistance between a connecting portion and a housing of the compressor when liquid refrigerant is collected in the housing is increased.